core: implement Kill/Automount/Mount Context/Runtime for io.systemd.Unit.List (#39391)

The PR implements the following objects + tests for
`io.systemd.Unit.List`:
- `KillContext`
- `AutomountContext`
- `AutomountRuntime`
- `MountContext`
- `MountRuntime`

It's a continuation of the following PRs:
* https://github.com/systemd/systemd/pull/37432
* https://github.com/systemd/systemd/pull/37646
* https://github.com/systemd/systemd/pull/38032
* https://github.com/systemd/systemd/pull/38212

src/test/test-varlink-idl-unit.c

@@ -1,7 +1,10 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include "automount.h"
 #include "cgroup.h"
 #include "ioprio-util.h"
+#include "kill.h"
+#include "mount.h"
 #include "numa-util.h"
 #include "process-util.h"
 #include "tests.h"
@@ -40,6 +43,9 @@ TEST(unit_enums_idl) {
         test_enum_to_string_name("slave", &vl_type_MountPropagationFlag);
         test_enum_to_string_name("private", &vl_type_MountPropagationFlag);
 
+        /* KillContext enums */
+        TEST_IDL_ENUM(KillMode, kill_mode, vl_type_KillMode);
+
         /* CGroupContext enums */
         TEST_IDL_ENUM(CGroupDevicePolicy, cgroup_device_policy, vl_type_CGroupDevicePolicy);
         TEST_IDL_ENUM(ManagedOOMMode, managed_oom_mode, vl_type_ManagedOOMMode);
@@ -47,6 +53,12 @@ TEST(unit_enums_idl) {
         TEST_IDL_ENUM(CGroupPressureWatch, cgroup_pressure_watch, vl_type_CGroupPressureWatch);
         TEST_IDL_ENUM(CGroupController, cgroup_controller, vl_type_CGroupController);
 
+        /* AutomountRuntime enums */
+        TEST_IDL_ENUM(AutomountResult, automount_result, vl_type_AutomountResult);
+
+        /* MountRuntime enums */
+        TEST_IDL_ENUM(MountResult, mount_result, vl_type_MountResult);
+
         /* UnitContext enums */
         TEST_IDL_ENUM(CollectMode, collect_mode, vl_type_CollectMode);
         TEST_IDL_ENUM(EmergencyAction, emergency_action, vl_type_EmergencyAction);